In an electrophotographic (EP) imaging process used in printers, copiers and the like, a photosensitive member, such as a photoconductive drum or belt, is uniformly charged over an outer surface. An electrostatic latent image is formed by selectively exposing the uniformly charged surface of the photosensitive member. Toner particles are applied to the electrostatic latent image, and thereafter the toner image is transferred to media, such as a paper substrate, intended to receive the final permanent image. The toner image is fixed to the media by the application of heat and pressure in a fuser assembly. A fuser assembly may include a heated roller and a backup roller forming a fusing trip through which the media passes. A fuser assembly may also include a fuser belt and an opposing backup member, such as a backup roller. Processing of substrates such as sheets of paper through the fusing trip compresses and flattens the sheet just before or as the image is being fixed onto the surface of the sheet.
Paper substrates are usually packaged in reams of 500 sheets enclosed in a protective, often waterproof wrapper. Since paper is somewhat hygroscopic, paper substrates may absorb moisture when exposed to ambient air. Depending on storage conditions for the paper substrates, once the protective packaging has been opened, the paper may absorb moisture from the surrounding air causing the fibers of the paper to swell and lengthen. This may result in a change in the dimensions of the paper substrates depending on whether the moisture is absorbed uniformly or non-uniformly across the length and width of each substrate. Such moisture absorption may lead to wavy edges being formed.
FIG. 1 illustrates a paper substrate 1 which has been exposed to a high level of ambient humidity on one end or edge 2, such as where a protective packaging for a ream from which the substrate 1 was taken has been opened only at one end. Thus, moisture was absorbed at the one exposed edge 2 creating a moisture gradient from the exposed edge 2 to a drier protected opposite edge 5 which was covered by the packaging. The moisture gradient caused the exposed edge 2 to lengthen in the width-wise direction, due to the swelling of the paper fibers. However, the substrate edge 2 is constrained by the dimensions of a dry portion 3 of the substrate 1 such that a boundary condition is essentially set up that will not allow the lengthened or widened edge 2 of the sheet to be substantially wider than the dry portion 3. Hence, the edge 2 becomes wavy or buckles due to this constraint and remains essentially the same effective horizontal width as the remainder of the substrate 1. In other words, for an 8.5″×11″ size substrate of paper, the distance between corners 4 and 4′ remains about 8.5″.
If the substrate 1 is fed with the wavy edge 2 first through a conventional fusing nip 6, which may be defined by a pair of fusing rollers, the edge 2 may be pressed out by the compressive forces applied by the nip 6, making the edge 2 flat, see FIG. 2. The edge 2 is now wider than the width of the dry portion 3, resulting in non-parallel outer edges 2A and 3A on each side of the substrate 1. Due to stress reactions in the non-parallel outer edges 2A and 3A of the substrate 1, corrugations 8 are formed in the substrate 1, see FIG. 3. The corrugations 8 are formed into wrinkles 8A as the substrate 1 passes through the nip 6. The wrinkles 8A cause defective copies and customer complaints.
U.S. Patent Application Publication No. US 2006/0133867 A1, the entire disclosure of which is incorporated herein by reference, provides one solution to this problem. U.S. patent application Ser. No. 11/468,516, entitled “Fuser Assembly Having Heater Element With Spaced-Apart Features”, filed on Aug. 30, 2006, the entire disclosure of which is incorporated herein by reference, provides another solution to this problem. Other solutions for reducing wrinkling in paper substrates having one or more wavy edges are desirable.